It is well known to store computer data on tape using a helical scan recording technique. Thus, for example, it is known to use DAT (Digital Audio Tape) technology to store computer data in accordance with the DDS (Digital Data Storage) format jointly developed by Hewlett-Packard Limited and Sony Corporation; the DDS format is specified in ISO/IEC Standard 10777:1991 E.
Data storage apparatus using DAT technology generally comprises:
a tape deck with a rotary head drum mounting heads for writing/reading data to/from tape in tracks of predetermined format; PA1 data organising means operative (i) during data storage to receive data to be stored and to organise it at least notionally, on the basis of frame units each corresponding to two track's worth of data, and (ii) during data retrieval, to disassemble the data out of frame units for output from said apparatus, said data organising means including a buffer memory for holding said frame units, PA1 formatting means operative (i) during data storage, to receive data transferred in from the buffer memory and to process it into said track format prior to being written in tracks to tape by said heads, constituent data elements from each frame unit being interleaved with each other across two tracks and (ii) during data retrieval, to receive data read from tape by the heads and to process it out of said track format ready for transfer out to buffer memory, the data in the bullet memory being held therein in non-interleaved form, PA1 transfer means for transferring data from the buffer memory to the formatting means during data storage and from the formatting means to the buffer memory during data retrieval, and PA1 a system controller for controlling and coordinating operation of the apparatus. PA1 a tape deck with a rotary head drum mounting heads for writing/reading data to/from the tape in tracks, PA1 data organising means operative (i) during data storage, to receive data to be stored and to organise it at least notionally, on the basis of frame units each corresponding to two track's worth of data and (ii) during data retrieval, to disassemble the data out of frame units for output from said apparatus, said data organising means including a buffer memory for holding said frame units, PA1 formatting means operative (i) during data storage, to receive data transferred in from said buffer memory and to process it into said track format prior to being written in tracks to tape by said heads, constituent data elements from each frame unit being interleaved with each other across two tracks, and (ii) during data retrieval to receive data read from tape by said heads and to process it out of said track format ready for transfer out to said buffer memory, the data in said buffer memory being held therein in non-interleaved form, PA1 transfer means for transferring data from said buffer memory to said formatting means during data storage and from said formatting means to said buffer memory during data retrieval, and PA1 a system controller for controlling and coordinating the operation of said apparatus; PA1 pass through the counter output of 2.degree. weighting onto the correspondingly weighted generator output, PA1 reconfigure the counter output of 2.sup.1 weighting onto the 2.sup.1 and 2.sup.2 weighted generator outputs, the 2.sup.1 weighted generator output being inverted or non-inverted in dependence on which track of each pair of tracks associated with a said frame unit, is being transferred by said transfer means, and PA1 reconfigure all other counter outputs onto generator outputs of one greater weighting. PA1 a subsequence register for holding a sub-sequence of said pre-determined sequence and having a plurality of outputs the states of which are determined by said sub-sequence and serve to define at least one randomisation/de-randomisation byte, PA1 first combinational logic connected to receive both a byte of data being transferred by the transfer means and a said randomisation/de-randomisation byte, and to combine these bytes to derive a randomised/de-randomised data byte, and PA1 second combination logic responsive to the states of said register outputs to generate a new sub-sequence for loading into the register, the new sub-sequence corresponding to the previous sub-sequence shifted by an amount corresponding to said at least one byte. PA1 byte output means responsive to the states of said register outputs to provide a plurality of randomisation/de-randomisation bytes sequentially arranged in said predetermine sequence, the byte output means including combinational logic for predicting bits of said randomisation/de-randomisation bytes not represented by the register outputs; and PA1 byte selection means for selecting from said plurality of randomisation/derandomisation bytes, the randomisation/de-randomisation byte to be combined by the first combinational logic with said byte of data, the byte selection means effecting its selection in dependence on which track of each pair of tracks associated with a frame unit, is being transferred by the transfer means.
For data storage apparatus implementing the aforesaid DDS format, during data storage the data organising means organises incoming data into groups of twenty two frame units known as G1 sub-groups, the final frame unit of a group including an index of record and file boundaries for data in the group. (It will be understood that the organisation of data into groups may be effected either by actual physical organisation of data within the buffer memory or by the appropriate setting of pointers delimiting group boundaries. Furthermore, the frame units themselves may not be specifically identified within a group but the group size and the manner of its processing within the data organising means is based upon the concept of a frame unit of data; for this reason, the data organising means can be said to effect organisation of data, at least notionally, on the basis of frame units). Once data has been assembled into a group, the frame units of the group are transferred to the formatting means and in the course of this transfer are randomized and interleaved. The formatting means thereafter carries out a number of functions including generating error correction codes from data, combining the data with the error correction codes and with auxiliary codes, generating track signals therefrom and passing the track signals to the head drum for writing to tape.
During data retrieval, the formatting means receives track signals read from tape and effects error detection/correction during processing of data out of said format. The read-back data is then transferred to the data organising means and in the course of this transfer is de-interleaved and de-randomized. The data organising means thereafter disassembles the data from the twenty-two frame-unit groups and outputs the retrieved data.
European patent specification EP-A2-0,437,779 (Archive Corporation) illustrates in general terms one form of DDS-based data storage device. In this case, there are separate read and write audio DAT formatter blocks each with its own local RAM memory.
Generally with regard to DAT-based storage apparatus, because of the complexity of the processing effected by the formatting means, there is a significant processing delay through the formatting means. As a result, it becomes necessary for data in respect of several tracks to be handled at any one time by the formatting means with the data associated with different tracks being at different stages of processing. Thus, it can be seen from a study of the timing diagrams of FIG. 4 of EP-A2-0,437,779 that the formatting means of the described storage apparatus has a processing propagation delay therethrough for writing/reading of at least ten times the time taken by the head drum to read/write a track. As a consequence of the large processing propagation delay, the formatting means requires a considerable amount of memory to hold all the data currently being processed; thus, for the apparatus shown in EP-A2-0,437,779 a minimum of six track's worth of data must be stored by each of the local RAM memories.
It is an object of the present invention to enable the processing propagation delay through the formatting means to be reduced.